It has been a while since you heard planetary scientists seriously talking about life on Mars. From the forbidding conditions revealed by the Viking landers of the mid-70s, to doubts about the origins of riverlike formations, to the meteoric rise and fall of the case for fossils in a Martian meteorite, researchers have learned the hard way not to get their hopes up.

So as several research teams announced over the past year that they had seen methane gas in the Martian atmosphere, the response has been so cautious that you'd hardly know just how revolutionary the discovery might be. For decades, methane has been near the top of scientists' list of biomarkers (substances whose presence is a possible sign of life). The idea of finding it on Mars seems so unlikely that many researchers assumed the discovery had to be some sort of mistake.

That reaction is no longer tenable. Last Thursday, at the annual meeting of the American Astronomical Society's Division for Planetary Science (DPS), planetary astronomer Michael Mumma of NASA's Goddard Space Flight Center announced hard-to-dispute evidence that the gas is really there. It may yet turn out to be nonbiological, but a living, breathing source is just as plausible. "I'll tell you quite honestly, I'm shocked," Mumma said to his colleagues. "We were not expecting this."

What makes methane so interesting is that the gas is unstable. On Earth, a methane molecule released into the air typically gets broken down by solar ultraviolet radiation in about 10 years. On Mars, farther from the sun, it lasts about 300 years. The persistence of the gas in our atmosphere indicates it is being replenished--in Earth's case, mostly by bacteria...